Rubber, each other get strengthen, and when the

Rubber, today, has been widely used in many different places and has fully changed our daily lives. It has been put into massive industry production due its low price and easy processing to obtain (Bai N. Y., 2008; Rongchun Z., 2013). Also its special chemical and physical property has the ability to reverse back to its original shape after a certain force is applied to it. The rubber’s molecular structure gives the reason of its property of reversibility, when the rubber is pulled, the strands which consist numerous small molecules crosslinked to each other get strengthen, and when the tension is released, the strands relax then the rubber return to its original shape. In the past few years, the self-healing materials gains more and more attention from the scientists around the world because of their ability to regress to their original state after damage (Jinrong W., 2017). The scientists first developed a kind of self-healing polymer which crosslinked by single reversible bonds which is weaker than the normal crosslinked covalent bonds (Rongchun Z., 2013). The new experiments, led by different groups of scientists, resulted with new solutions for stronger bonds by using the concept of supramolecular structure which is a reversible structure of small molecules that are held together by relatively weak bonds compared to covalent bonds such as hydrogen bond (Cordier P., 2008; Nicholas J. T., 2005; Tro, 2017). In order to understand about the self-healing rubber, one must first recognize the idea how the it is formed and the concept behind that. The self-healing rubber can be formed in a few possible ways. In all this field of studies, there are three notable groups of scientists. A group of scientists at the Industrial Physics and Chemistry Higher Educational Institution in Paris led by Ludwik Leibler first introduced the supramolecular structure into the rubber which is made with fatty acids(from vegetable oils) and urea (Bai N. Y., 2008; Cordier P., 2008). The supramolecular network in this case can be formed by mixing the ditopic and multitopic molecules which are able to connect to more than two other molecules by directional interaction (Cordier P., 2008). Figure 1. Supramolecular Structure (Cordier P., 2008)The Figure shown on the left is an example of reversible supramolecular structure that the scientists given in the view of molecular scale. The red dots are representing the tritopic molecules and the blue dots are the the ditopic molecules.  And the dotted lines show the directional interactions between the molecules. They used a two-step process to synthesize strong multiple hydrogen bonds in the material. First, they used carboxylic-acid, and condense it with the diethylene triamine makes it become oligomers. Then they react the products from last step with urea. In the second step, nitrogen-containing group is add onto the compound which makes it the ability to form many hydrogen bonds between the molecules (Bai N. Y., 2008). They obtained a viscoelastic material, they plasticize the compound with dodecane resulting a rubber-like material. But this new material which has not named yet is different from the normal rubber is that when the material is cut, the broken surface has a lot numerous non-associated groups wanted to reform hydrogen bonds. So when the two broken surfaces are brought back together, the hydrogen bonds reformed and the material healed itself without any external pressure or adding any chemical substances into the material. The reformed material can can stretch five times as big as its original size. To test how the new material’s strength, Ludwik set the material after cut into different conditions to see how it behaves (Figure 1). Figure 1. Self-healing in different time frame (Cordier P., 2008)The diagram shows the materials behaviors under certain temperature and time frame.  Graph a shows the stress-strain behaviors of the material after it being cut and put back together within 5 min at room temperature (20 ?). Graph b shows the stress-strain behaviors of the material after leaving the cut for 6 hours, and graph c shows the stress-strain behaviors after leaving the cut for 18 hours. From figure 4.a,b,c the self-healing can be still efficient after 18 hours, and the self-healing is even possible after more than a week. By using the method they concluded, it’s very easy and inexpensive to make the material, and all the raw material as stated earlier is renewable and recyclable, so it’s “green” which means friendly to the environment (Bai N. Y., 2008). After Leibler’ group, another group of scientists followed their work. Instead of simply use the hydrogen bonded molecular network which is formed with oligomers which discussed earlier, another group of scientists from American Chemical Society also introduced the concept of heterogeneous structure (Bai N. Y., 2008; Cordier P., 2008;  Rongchun Z. 2013). Their work is focusing on investigating the heterogenous structure and hydrogen bonding dynamics as well as its property of aging in self-healing rubber. As stated earlier, hydrogen bond is really important to the self-healing rubber, it determines self-healing rubber’s ability to heal itself. The reason that this rubber is called heterogenous structure, is because that by their experiments, they found out that the self-healing rubber basically has two component systems, about 85% of the it rich in hydrogen bonded crosslinked structure and associated molecules, this structure is able to a glass transition at the room temperature. The glass transition is a reversible transition that takes place in amorphous materials as well as the material that doesn’t have an exact shape. And the other 15% of the material is made up of mobile aliphatic chains (Cordier P., 2008;  Rongchun Z. 2013). In 2017, a group of researchers lead by Cai, together with Jingrong Wu from Sichuan University  addressed another advanced technique of forming self-healing rubber. They made a step further from the previous works by introducing covalent bonds into the rubber which has not been done before. The problem was that the hydrogen bond is polar motifs however the covalent bond is nonpolar-motifs (Jinrong W., 2017). Since one is polar and the other one is non-polar, they won’t mix together normally without any co-solvents. So the scientists first came up with a theory to mix those two bonds together. And from the theory, they developed a polymers called randomly branched polymers that can force the unmixable bonds to be mixed homogeneously on the molecular level (Jinrong W., 2017; John A., 2017). And this polymers are able to form a supramolecular network connecting with that homogenous bonds. To test the rubber’s characteristics, the scientists used a test called uniaxial tensile test, stretch the rubber at a certain speed of 0.014/second. And they found out the rubber fractured 13500 J/m^2 of energy is put on the rubber (Jinrong W., 2017). The normal rubber cracks when the force that applied to the rubber reaches its maximum. The new self-healing rubber developed  so-called “macro-crazes” that these crazes can evenly distribute the force that it is applied onto the rubber so there won’t be any localized point causes the cracking happens, when the tension is released, the rubber can go back to the original shape and the crazes will heal themselves because of all the reversible bonds. This new rubber is just one another small step in the history of material, but this topic is important because it can be used in many different aspects and really change our daily lives. This new material can extend the lifetime of the rubber products that are being using in many fields such as automotive, and biology as well. For examples, the new rubber can be put into manufacturing to make artificial human bones, stretch-proof car or kid’s toy. As go through the paper, one can find out the development of this new rubber, the usage of  more and more advanced methods. For the further study, the scientists can study the way to make the rubber heal itself faster and stronger, and have a better understanding its toughness, and self-healing ability (John A., 2017). There is still a lot to study in the field of material science.


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